The present invention relates to a method for compensating an operating error of a clock, particularly that of transmission clocks used in the base stations of a long-range paging system.
A problem in long-range paging systems with a dense network of base stations is that a paging receiver may be located within the audible range of two transmitters. Since transmitters operate on the same frequency, they interfere with each other unless synchronized.
Synchronization means that the transmission clocks of base stations send out the same information at exactly the same instant, so-called quasi synchronous transmission. In long-range paging systems with digital information, this means that the same information symbol is transmitted from various base stations at exactly the same time. Transmission is quasi synchronous if the phase difference of symbols transmitted by various base stations, upon the arrival thereof in a paging receiver, does not exceed 1/4 of the time required by transmission of the symbol.
As transmission speed increases, the requirement for synchronization will be stricter since the duration of a symbol becomes shorter.
Especially in the novel long-range paging system according to ERMES standard there is a set requirement that no more than .+-.5 microseconds difference can be accepted in various base stations at the commencement of symbol modulation.
In order to attain a synchronization accuracy of five microseconds, a base station must necessarily be provided with a high-precision time reference (atomic clock), which is synchronized to a certain time reference, or with continous reception of time from a high-precision time reference. A drawback with both of these is a high price. High-precision time references are expensive and, on the other hand, the attainment of a necessary degree of precision would require the use of a satellite receiver and those are also expensive.
A preferred solution, durrently used for synchronization, is to provide a base station with a quartz oscillator as time reference and to synchronize the clocks of various base stations periodically to each other by using a radio path for the transmission of a synchronization signal. A problem here is, however, that during the course of synchronization the transmission of pagings is prevented. Synchronization takes about a minute. The synchronizing accuracy of .+-.5 microseconds would require synchronization at least at five-minute intervals due to the insufficient accuracy of a quartz oscillator. However, such a frequent synchronization takes up too much radio capacity to be practical.